The thing about Bayonetta is that she's almost never just firing her guns - she's doing other things such as kicking and twirling that get off most of the hits.
Most fighters aren't designed around platforming, though. Most fighters are designed around a single, flat plane in an enclosed space. Smash is not. You aren't dealing with the same set of game design constraints, and you have to take that into consideration when designing a moveset. Saying "most fighters outside of Tekken have gun characters" ignores that.
Well sure, those fighters aren't designed around platforming, but the issues and solutions with animations and projectile and such carry over, as with most animation principles of fighting games.
Those solutions don't just disappear due to a genre shift, especially considering most of the solutions for gun inclusions have already been implemented in other Smash characters
-low damage (Star Fox, Bayonetta)
-muzzle flash hitbox only (Lucario)
-ammo system (Project M Snake, Robin)
-long telegraphed draws (All the Star Fox characters, Bayonetta, K Rool)
-big recoil (Falco, Wolf, Bayonetta, K Rool)
-unorthodox firing angles (Bayonetta)
-exaggerated gun sizes (Falco, Wolf, Bayonetta, K Rool)
-short bullet range (All the Star Fox characters and Bayonetta)
-slow bullet speed (Zero Suit Samus, K Rool)
Notice that none of these are reliant on typical fighter conventions like closed stages, health bars or special meters.